The book Philosophy of Mathematics by Øystein Linnebo, is a nice, systematic and accessible introduction to the philosophy of mathematics by a leading scholar in the field.
Plato's Problem: An Introduction to Mathematical Platonism by M. Panza and A. Sereni.
A Subject With No Object: Strategies for Nominalistic Interpretation of Mathematics by John P. Burgess and Gideon Rosen. A book on the strategies of various nominalists in mathematics who have attempted to "purify" mathematics from abstract objects, and a critical assessment of such efforts by the authors.
SEP and IEP are good sources of information about philosophy.
PhilPapers is a comprehensive index and bibliography of philosophy, maintained by the philosophy community.